Recent Accounting Pronouncements Recent Accounting Pronouncements (Narrative) (Details) - Adoption of ASU 2016-09 [Member] - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016
Recent Accounting Pronouncements
Change in accounting policy, description		Upon adoption of ASU 2016-09, the Company elected to change its accounting policy to account for forfeitures as they occur; this change was applied using the modified retrospective transition method with a cumulative effect adjustment
Excess tax benefit from share-based compensation	$ 9.2	$ 32.5
Reclassification or adjustment due to the adoption of a accounting pronouncement [Member]
Recent Accounting Pronouncements
Cumulative effect adjustment to opening Retained earnings	$ (0.3)	$ (0.3)
Tax withholdings for share-based compensation			$ 0.2
Excess tax benefit from share-based compensation			$ 30.1
Effect on diluted earnings per share	$ 0.09	$ 0.32

Amount of cash outflow for realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

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